Not a great assumption on your part. Since you did not lead trump S3 assumes you are weak and he does not know where the Right is. So not surprising he is not leading trump on trick 2. You should not be so surprised, IMO. I think your 9D lead is okay. You need to get two tricks with your J/K.
I have no power in off suits, but I have a double diamond. I lead

, which my partner takes.
He doesn't lead trump back, so I assume he has none.
I take 2nd trick with

. Knowing I have a sure point, I lead a junk card back to him, which, to my surprise, he trumps. He now leads

back to me, and I play the

. He trumps my last lead with the

for a surprising 2 points.
He didn't think I had the right, since I didn't lead it. I told him I didn't think he had any, since he didn't lead it back, and that the rule of thumb is to not lead the right on a next call.
Should I have led the

? I did not want to strip him of trump, and with no power in my hand, I wanted back in the hand with the other diamond.